An artist's concept of the GRAIL mission. The twin spacecraft will fly in tandem orbits around the Moon for several months to measure its gravity field in unprecedented detail and unlock clues about what lies beneath the lunar surface.
Credit: NASA/JPL
WASHINGTON: On Saturday, NASA launched a pair of twin satellites that will fly in tandem around the Moon and map its core for the first time.
The $500 million Gravity Recovery and Interior Laboratory spacecraft, dubbed GRAIL, will use high-quality gravity field mapping to determine the Moon's interior structure.
"We have used gravity science before to try to gain some insight as to what is going on inside the Moon, however these have been very primitive attempts," said program scientist Robert Fogel of NASA.
"If those previous attempts could be likened to a magnifying glass, GRAIL by contrast would be a high-powered microscope."
Smooth launch following delays
High upper level winds delayed the first attempt on Thursday, and also briefly set back Saturday's launch. But once the rocket blasted off, the journey went smoothly and the twins separated as planned just under an hour-and-half into the flight.
The duo will travel on a three-month route to the Moon, arriving into a polar lunar orbit around New Year's Day, and flying roughly 55 km above the lunar surface.
With one spacecraft trailing the other, the plan is to use a microwave ranging system that will precisely measure the distance between the two spacecraft.
By watching that distance expand and contract as the pair fly above the lunar surface, researchers can map the Moon's underlying gravity field, revealing the contents of the inner core of the Moon.
Did the Moon have a sister?
The mission should also shed light on the unexplored far side of the Moon, and perhaps tell scientists whether there was once a second Moon that fused with ours.
Scientists believe that the Moon was formed when a planet-sized object crashed into the Earth, throwing off a load of material that eventually became what we now recognize as our planet's airless, desolate satellite.
The Moon's near side, facing us, is dominated by vast smooth 'seas' of ancient hardened lava, while the far side, in contrast, is marked by mountainous highlands.
To account for this difference in landscape, a second moon hypothesis was recently published in the journal Nature. It has not yet been proven but has garnered widespread attention.
"These measurements will tell us a lot about the distribution of material inside the Moon, and give us pretty definitive information about the differences in the two sides of the Moon's crust and mantle," said David Smith, deputy principal investigator for the GRAIL mission at the Massachusetts Institute of Technology in Boston.
If the density of the crust's material on the lunar far side differs from that on the near side in a particular way, the finding will lend support to the 'two moon' theory, he added. The mission will also provide scientists with a better understanding of how Earth and other rocky planets may have once formed.
Temperature is key to Moon's evolution
"If we want to reconstruct the evolution of the Moon over time, we certainly need to reconstruct the temperature structure of the Moon right now," said principal investigator Maria Zuber of MIT.
Little is known for certain about what lies inside the lunar body. The widely held belief that there is a small solid iron core surrounded by a liquid iron core is unproven, said Zuber.
"It is actually quite possible that deep inside the Moon the core could be titanium oxide, which is a material that would have fallen out or would have crystallised out of the magma ocean and sunk to the deep interior of the Moon," she said.
Once the GRAIL twins enter the orbit of the Moon, they will line up with each other and "essentially chase each other around in a polar orbit as the Moon rotates slowly underneath them," said Zuber.
Next up for NASA is the launch of its Mars Science Laboratory in November on a nearly two-year journey to the red planet.
